An oil tanker glides under San Francisco’s Golden Gate Bridge, one of the many ships arriving daily in this busy port. Large ships can carry 20 million gallons of ballast for stability in open waters. Traveling globally from port to port they fill and dump their water, spreading microscopic organisms as they go. Biologists have found hundreds of invasive species in San Francisco Bay, which Biologist Andrew Cohen of the Center for Research on Aquatic Bioinvasions, says makes it one of the most invaded estuaries in the world. The Bay averages one invasive species introduced every 24 weeks since 1970, says Cohen.

Overall, 84 percent of the world’s coasts are being affected by foreign aquatic species, according to a Nature Conservancy report.

California is cracking down on invasive species, and that could have a big impact on national regulations. The state has passed the strictest rules in the country to prevent cargo ships from bringing foreign plants and animals to San Francisco Bay. But the standards are so firm that California may not be able to enforce them.